1. Field of Invention
The present invention relates to a structure for a read-only-memory (ROM), and more particularly to a structure for a ROM having both bipolar and channel transistors.
2. Description of Related Art
Read-only-memory has been extensively used in digital devices like mini computers, and microprocessor systems, for storing invariable programs for system operation. Since read-only-memory is invariable, usually a client first sends his program to a ROM manufacturing factory, then the program is built or coded into a ROM. After that the ROM is finished and sent to the client. The ROM manufacturing process is very complicated, involving many steps, each of which takes time and must be processed and controlled precisely. Because most ROM elements have the same structure but only differ in the data stored therein at the programming stage, process steps before the programing stage can be performed to produce half-finished products. When ROM elements are ordered by the client for storing a specified program, a mask for the program can be rapidly produced. The half-finished ROM elements can then be programmed and sent to the client as soon as possible. Therefore, post-programmed mask ROMs have become widely used in this art.
Conventional ROMs use channel transistors as memory cells. In the programming stage, impurities are selectively implanted into prescribed channel regions, to change their threshold voltage, and thus control the conduction of the channel transistors. FIG. 1 (Prior Art) shows a cross-sectional view of a conventional ROM using channel transistors. First, N-type impurities such as arsenic (As) are implanted into a silicon substrate 10 to form a plurality of equally spaced buried bit lines 11. Channel regions are simultaneously formed between the buried bit lines 11. Next, an oxidation step is performed. Due to different oxidation speeds, thick isolation layers 12a are formed on the buried bit lines 11 and thin oxide layers 12b are formed on the channel region. Then, a polysilicon layer is deposited and etched according to a pre-defined pattern to form word lines 13 across bit lines 11, so as to construct channel transistors. After that, a mask layer 14 is formed thereon and then etched to expose predetermined channel regions 15 which will be coded. P-type impurities, like boron, are implanted to complete the programming step.
The above-described conventional ROM arrangement occupies a large space because it uses channel transistors as memory cells. Additionally, impurities tend to diffuse in the programming step. As a result, the dimensions of ROM elements can not be reduced, which limits down-sizing of the ROM elements, and restricts the density of ROM elements per each unit area.